1. Field of the Invention
The present invention relates to a system and a method for generating a three-dimensional (3-D) image of an object by using two-dimensional (2-D) images of the object; and, more particularly, to a system and a method for generating a 3-D image by fast computing the distance between adjacent 2-D images.
2. Description of the Related Art
Ultrasound imaging systems are gaining popularity in medical diagnostics and non-destructive inspection fields. In ultrasound imaging systems, an array of transducers or a probe generate and transmit an ultrasound signal and receive the ultrasound signal reflected from a target being scanned. The received ultrasound signal is processed to display an image of the target.
Two-dimensional (2-D) ultrasound imaging systems generate a 2-D image of a target by transmitting and receiving an ultrasound signal in a single scan plane. The generated 2-D image represents only one cross-section of the target. Thus, the overall structure or shape of the target is not readily recognized.
In order to produce a 3-D image of the target, a series of 2-D images were obtained by moving a probe in a predetermined direction over the target. FIG. 1 shows a series of 2-D image frames of a target, each representing a different cross-section of the target. The frame of reference is an orthogonal depth-lateral-elevation (X-Y-Z) coordinate system 101. X-axis represents the scanning depth; Y-axis represents the direction of arrangement of xe2x80x9cmxe2x80x9d transducers 1081 to 108m; and Z-axis represents the moving direction of a probe 108 or an array of transducers 1081 to 108m. The probe 108 can generate a 2-D image frame 1101 by transmitting and receiving an ultrasound signal and processing the ultrasound signal reflected from the target to be examined. Repeating the above steps by moving the probe along Z-axis, a series of 2-D image frames 1101 to 1103 of the target could be obtained. A 3-D image of the target would be produced by synthesizing the series of 2-D image frames 1101 to 1103.
Producing the 3-D image in a manner described above requires accurately measuring distances between adjacent 2-D image frames 1101 to 1103. The distance between a pair of adjacent frames is computed as the product of the velocity and moving period of time of the probe 108 between the pair. When the user moves the probe 108 manually, however, it is difficult to move the probe 108 with a constant velocity. Without knowing the velocity of the probe 108, it is not possible to accurately calculate the distance even if when 2-D images were generated is known.
To address this problem, xe2x80x9ca speckle noisexe2x80x9d appearing on the respective 2-D image frames has been taken advantage of estimating the distance. The speckle noise means a noise appearing on a screen as a speckle, which is caused when objects in the scanning area smaller than the wavelength of an ultrasound signal disperses or interferes with the reflected signal. Since speckles in image frames have similar, as the image frames are closer to each other, the correlation of speckles in two different image frames could be used to estimate the distance between the two image frames.
For the speckle correlation, an experimental phantom, composed of a material whose speckle characteristics are known, is first prepared; and then a series of 2-D image frames for the experimental phantom are produced by moving a probe at a predetermined velocity over a predetermined distance. Now, distances between adjacent 2-D image frames are known, it is possible to obtain relationship between speckle correlation and distance by computing the speckle correlation between a pair of image frames. FIG. 2 illustrates the speckle correlation as a function of the distance. In FIG. 2, xcfx81 is a speckle correlation and d is a distance between consecutive two 2-D image frames.
Armed with the function, a series of 2-D image frames of an actual target are generated. And, speckle correlation between each adjacent pairs of the 2-D image frames is calculated. Finally, the distance between them is estimated by using the experimentally obtained speckle correlation/distance function. Thereafter, the series of 2-D image frames are compiled by using the estimated distances, to finally produce a 3-D image of the target.
For example, the 2-D image frames are composed of not only speckle regions but also of regions representing inner structures or shapes of the target. In order to estimate distances between all the pairs of adjacent 2-D image frames by using the speckle correlation computed as described above, it is required to extract the speckle regions from them. FIG. 3 exemplifies consecutive two 2-D image frames of a target, e.g., ith and (i+1)th image frames, each image frame being divided into a plurality of sub-blocks to identify regions having speckles. The speckle regions are denoted as dark squares. According to the method described above, the speckle correlation between ith and (i+1)th image frames is calculated by using sub-blocks having speckle noise only.
However, when the target has substantially different characteristics from the experimental phantom, which is to be scanned, the estimated distance between two 2-D image frames of the target would have errors. Also, it is difficult to extract the speckle regions from the respective 2-D image frames. Furthermore, it is time-consuming to extract the speckle regions from the respective 2-D image frames.
It is one objective of the present invention to provide a method for estimating, simple and accurately, the distance between adjacent 2-D image frames obtained by moving a probe manually in a predetermined direction.
It is the other objective of the present invention to provide a system and a method for generating a 3-D image by fast computing the distance between adjacent 2-D image frames, which are obtained by moving a probe in a predetermined direction over a target to be examined.
In accordance with an aspect of the present invention, there is provided a method for calculating a distance between consecutive two image frames obtained by moving a probe over a target to be examined, wherein the probe transmits and receives ultrasound signals, comprising the steps of: a) producing a first main frame, a second main frame parallel to the first main frame, and a supplementary frame inclined at an angle with respect to the first main frame; b) creating a virtual frame parallel to the first main frame by using the first main frame and the supplementary frame; c) calculating a first correlation coefficient between the first main frame and the virtual frame; d) computing a second correlation coefficient between the first and second main frames; and e) estimating a first distance between the first and second main frames by using the first and second correlation coefficients and a second distance between the first main frame and the virtual frame.
In accordance with another aspect of the present invention, there is provided a method for producing a 3-D image of a target to be examined, comprising the steps of: a) generating pairs of image frames by moving a probe over the target; b) creating a virtual frame parallel to the main frame by using the main and supplementary frames; c) calculating a first correlation coefficient between the main frame and the virtual frame; d) computing a second correlation coefficient between the main frame and a next main frame adjacent to the main frame; e) estimating a first distance between the main frame and the next main frame by using the first and second correlation coefficients and a second distance between the main frame and the virtual frame; f) repeating the steps a) to e) for the remaining main frames of the pairs; and g) producing a 3-D image of the target by using the first distances for all the main frames of the pairs.
In accordance with yet another aspect of the present invention, there is provided a system for producing a 3-D image of a target to be examined, comprising: a probe for generating pairs of image frames by moving a probe over the target; means for calculating distances between the main frames of the pairs; and means for displaying a 3-D image of the target produced by using the distances, wherein the means for calculating the distances includes: means for creating a virtual frame by using the main and supplementary frames; means for calculating a first distance and a first correlation coefficient between the main frame and the virtual frame, and a second correlation coefficient between the main frame and an adjacent main frame; and means for estimating a second distance between the main frame and the adjacent main frame by using the first distance and the first and second correlation coefficients.